Not clear. To me. Is this an indicator for RPM like a tachometer or some kind of engine wear counter?
How important is accuracy? If it is “just” RPM this is feasible if you can live with some error, still feasible if you want higher accuracy or perhaps most rapid response. But a bit more challenging.
Picking up the hall effect and processing that no large deal.
Control little brushless motor like in r/c cars or quadcopters no large deal.
Getting a good rapid and faithful tracking of the one to the other is more of a challenge but nothing that hasn’t been done before.
a7